Pure functions concept in Ruby - Step-by-Step Execution

Concept Flow - Pure functions concept

Call function with input

↓

Compute output using input only

↓

Return output

↓

No side effects

↓

Same input => Same output every time

A pure function takes input and returns output only based on that input, without changing anything outside or depending on outside state.

Execution Sample

Ruby

def add(a, b)
  a + b
end

result = add(2, 3)
puts result

This code defines a pure function 'add' that returns the sum of two numbers and prints the result.

Execution Table

Step	Action	Input	Computation	Output	Side Effects
1	Call add function	a=2, b=3	Calculate 2 + 3	5	None
2	Return from add	a=2, b=3	Return 5	5	None
3	Print result	result=5	Output 5 to console	5	Console output
4	End	-	-	-	No side effects in function

💡 Function ends after returning output; no side effects occurred inside the function.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	-	2	2	2	2
b	-	3	3	3	3
result	-	-	5	5	5

Key Moments - 3 Insights

Why does the function always return the same output for the same inputs?

Because the function uses only its input parameters to compute the output, as shown in execution_table step 1 and 2, it does not depend on or change anything outside.

Does the function change any variables outside itself?

No, the function has no side effects. The variable_tracker shows only internal variables and the result variable outside is assigned after the function returns.

What counts as a side effect in this example?

Printing to the console is a side effect, but it happens outside the function after it returns, as shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of the function at step 2?

A 5

B 2

C 3

D nil

💡 Hint

Check the 'Output' column at step 2 in the execution_table.

At which step does the function produce a side effect?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Side Effects' column in the execution_table.

If the function used a global variable inside, how would the output behavior change?

A Output would always be the same for same inputs

B Function would have no output

C Output might change even with same inputs

D Function would stop working

💡 Hint

Pure functions depend only on inputs; using global variables breaks this, as explained in key_moments.

Concept Snapshot

Pure functions:
- Always return same output for same inputs
- No side effects (no changing outside state)
- Output depends only on input parameters
- Easy to test and predict
- Example: def add(a,b); a+b; end

Full Transcript

This visual execution shows how a pure function works in Ruby. The function 'add' takes two inputs and returns their sum. It does not change anything outside itself, so it has no side effects. The execution table traces each step: calling the function, computing the sum, returning the result, and printing it. Variables 'a' and 'b' keep their values, and 'result' stores the output. Key moments clarify why the output is always the same for the same inputs and what counts as side effects. The quiz tests understanding of output, side effects, and the impact of using external variables. Pure functions are simple, predictable, and reliable building blocks in programming.

Practice

1. Which of the following best describes a pure function in Ruby?

easy

A. A function that reads user input and prints output directly.

B. A function that always returns the same output for the same input and has no side effects.

C. A function that modifies global variables to keep track of state.

D. A function that changes the value of its arguments.

Solution

1. Step 1: Understand pure function definition

   A pure function always returns the same result when given the same input and does not affect anything outside itself.

2. Step 2: Compare options to definition

   A function that always returns the same output for the same input and has no side effects. matches this definition exactly. Options A, B, and D involve side effects or changing external state.

3. Final Answer:

   A function that always returns the same output for the same input and has no side effects. -> Option B

4. Quick Check:

   Pure function = consistent output + no side effects [OK]

Hint: Pure functions never change outside data and always return same output [OK]

Common Mistakes:

• Thinking functions that modify variables are pure
• Confusing input/output consistency with side effects
• Assuming printing output is allowed in pure functions

2. Which Ruby function below is a pure function?

easy

A. def add(a, b); a + b; end

B. def add(a, b); puts a + b; end

C. def add(a, b); @sum = a + b; end

D. def add(a, b); print a + b; end

Solution

1. Step 1: Check each function for side effects and output consistency

   def add(a, b); a + b; end returns sum without changing anything outside or printing. def add(a, b); puts a + b; end prints (side effect). def add(a, b); @sum = a + b; end changes instance variable (side effect). def add(a, b); print a + b; end prints (side effect).

2. Step 2: Identify pure function

   Only def add(a, b); a + b; end meets pure function rules: same output for same input, no side effects.

3. Final Answer:

   def add(a, b); a + b; end -> Option A

4. Quick Check:

   Pure function = no side effects + returns value [OK]

Hint: Pure functions return values without printing or changing variables [OK]

Common Mistakes:

• Confusing printing output with returning value
• Assuming modifying instance variables is allowed
• Assuming print doesn't cause side effects

3. What is the output of this Ruby code?

def multiply(x, y)
  x * y
end

result = multiply(3, 4)
puts result

medium

A. Error

B. nil

C. 12

D. 7

Solution

1. Step 1: Understand the multiply function

   The function returns the product of x and y, so multiply(3, 4) returns 12.

2. Step 2: Check the output statement

   puts prints the returned value, so it prints 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   multiply(3,4) = 12 [OK]

Hint: Pure functions return predictable output; multiply(3,4) = 12 [OK]

Common Mistakes:

• Thinking function returns nil if no explicit return
• Confusing addition with multiplication
• Expecting error due to missing return keyword

4. Identify the error in this Ruby function that is supposed to be pure:

def increment(n)
  n += 1
  puts n
end

medium

A. It modifies the input variable n.

B. It uses invalid syntax for increment.

C. It does not return any value explicitly.

D. It prints output, causing a side effect.

Solution

1. Step 1: Check for side effects

   The function prints the value of n, which is a side effect because it affects outside world.

2. Step 2: Verify other aspects

   Modifying n locally is allowed; syntax is correct; implicit return is allowed but printing breaks purity.

3. Final Answer:

   It prints output, causing a side effect. -> Option D

4. Quick Check:

   Pure functions must not print or cause side effects [OK]

Hint: Printing inside function breaks purity due to side effects [OK]

Common Mistakes:

• Thinking local variable change breaks purity
• Assuming implicit return is an error
• Ignoring side effects caused by printing

5. You want to write a pure function in Ruby that takes a list of numbers and returns a new list with each number doubled. Which implementation is pure?

hard

A. def double_list(nums)
nums.map { |n| n * 2 }
end

B. def double_list(nums)
nums.collect! { |n| n *= 2 }
nums
end

C. def double_list(nums)
nums.map! { |n| n * 2 }
end

D. def double_list(nums)
nums.clear
nums.push(2, 4, 6)
end

Solution

1. Step 1: Understand pure function requirements

   A pure function returns new data without changing input or external state.

2. Step 2: Analyze each option

   def double_list(nums) nums.collect! { |n| n *= 2 } nums end uses collect! which modifies original array (side effect). def double_list(nums) nums.map { |n| n * 2 } end returns a new array with doubled values without changing original. def double_list(nums) nums.map! { |n| n * 2 } end uses map! which modifies original array (side effect). def double_list(nums) nums.clear nums.push(2, 4, 6) end clears and modifies original array (side effect).

3. Final Answer:

   def double_list(nums) nums.map { |n| n * 2 } end -> Option A

4. Quick Check:

   Pure function = no input change + returns new data [OK]

Hint: Use non-destructive methods like map, avoid modifying input [OK]

Common Mistakes:

• Using destructive methods like map! or clear
• Modifying input array instead of returning new one
• Confusing each with map for transformation